In traditional extrusion processes a billet of a single type of material is pushed and/or drawn through a printhead to create a rod, rail, pipe, or other similar structure. Various applications leverage this capability. For instance, extrusion can be used with food processing applications to create pasta, cereal, snacks, etc.
However, conventional extrusion techniques are limited. For example, conventional techniques cannot render relatively high aspect-ratio, fine featured (e.g., less then 5 micron) porous (e.g., 0.01 mm RMS) structures at an economical cost. Thus, conventional extrusion typically is not used for creating conducting contacts and/or channels for electrochemical (e.g., fuel), solar, and/or other types of cells, which leverage high aspect-ratio fine featured porous structures to increase efficiency and electrical power generation.
Coextrusion, generally, is the process of extruding two or more different materials through a printhead die to create a coextruded structure. For example, extruded flows of first and second inks are merged into a single flow in which the second ink surrounds the first ink. The single flow is then applied to a substrate to produce at least one composite material. Coextrusion can be used, for example, to create high aspect-ratio, micron level sized, structures for use in solar cells or fuel cells. See U.S. Patent Application Publication No. 2007/0108229, the disclosure of which is incorporated herein in its entirety.
Unfortunately, coextrusion processes suffer from several disadvantages. Specifically, when two dissimilar materials are coextruded they may mix at their interface. Such mixing is undesirable because it degrades the printing resolution and also lowers the aspect ratio of the printed structure. Additionally, coextruded structures may not achieve high aspect-ratios unless the coextruded materials have a sufficiently high yield stress or viscosity, such that the coextruded structure does not slump under the effects of, for example, gravitational force, in the timescale needed for processing. The sufficiently high yield stress ensures that for stresses below this limit, the ink behaves as a solid instead of a liquid, causing it to maintain its shape. An ink with a sufficiently large viscosity will also maintain its shape, provided that the timescale between printing and post-processing (e.g. drying, firing, etc) is short compared to the timescale of viscous relaxation.
In order to overcome this disadvantage, inks having high yield stresses or very high viscosities can be used. A high yield stress or high viscosity reduces mixing between the two inks and enables high aspect-ratios, but unfortunately introduces further disadvantages. An ink having a high yield stress or high viscosity will experience a large drop in pressure as it flows through the fluidic channels of the coextrusion device. Such materials thus generally require high pressure operation. However, low pressure operation of the coextrusion device is desirable because it enable a wider set of construction techniques for the printhead of the coextrusion device, as well as reducing the amount of wear on the printhead during operation. Furthermore, an ink which posseses a high yield stress and/or a high viscosity may be more likely to clog the printhead than an ink that flows with an arbitrarily low shear stress.
Therefore, there is a need in the art for a coextrusion system that comprises inks having low yield stress values or low viscosities, and precludes mixing of the inks or clogging of the printhead while enabling high aspect-ratio coextruded structures.